Solar light collecting method in multi-tower beam-down light collecting system

ABSTRACT

It enhances heat collecting efficiency of sunlight received by heliostats. It is a solar light collecting method in a multi-tower beam-down light collecting system, including a tower selection. The multi-tower beam-down light collecting system is a system in which, in a field where a plurality of beam-down light collecting towers are present, light primarily reflected by heliostats  1  around each tower  4  is secondarily reflected by a reflector  3  at a top part of the tower  4  and is collected on a receiver  3  on the ground, and the tower selection is a process in which, assuming that the heliostat  1  in a given position receives sunlight and reflects the sunlight toward each of optionally selected two of the towers  4, 4 , a light receiving quantity on the receiver  3  of each of the towers  4  is compared, and one of the towers  4  in which the light receiving quantity is relatively large is selected to reflect the sunlight toward the one of the towers  4.

TECHNICAL FIELD

The present invention relates to a method of enhancing light collectingefficiency of solar energy in a multi-tower beam-down light collectingsystem.

BACKGROUND ART

Of renewable natural energy, solar thermal energy is very promising asenergy to replace fossil fuel, owing to its abundant potential quantity(potential quantity of energy resource). The intensity of solar thermalenergy, though it varies depending on locations, is about 1 kW/m².Thermal energy of sunlight can be sufficiently utilized as an energysource for operating a thermochemical reaction plant, a power generationplant or the like. In order to utilize the solar thermal energy as anenergy source, it is required to be efficiently converted into chemicalenergy or electric energy, and in order to enhance the conversionefficiency, it is required to efficiently collect the sunlight.

The position of the sun relative to a point on the ground changes overtime due to the rotation of the earth. Therefore, in order to collectthe sunlight and to collect solar energy efficiently, it is required totrack the sun. A device for tracking the sun is called a heliostat.

In conventional heliostat sun-tracking-systems, a centralized control ofeach heliostat through wire and wireless communication, or a control bymeans of optical sensors have been performed.

In order to collect sunlight and to efficiently obtain the thermalenergy, it is required to make the heliostats accurately track theposition of the sun. The energy obtained by collecting the sunlight istheoretically proportional to the total area of mirror surfaces of theheliostats. Therefore, an issue in installing the heliostats is that, inorder to obtain large quantity of energy, it is necessary to increasethe mirror surface area of the heliostats or to increase the number ofheliostats.

When obtaining thermal energy of collected sunlight using a large numberof heliostats, it is necessary to make each heliostat track the sun, andto concentrate the reflection light of the sunlight received in eachheliostat at one point while controlling the orientation of eachheliostat.

Meanwhile, systems for collecting the sunlight received in heliostatsare classified broadly into a tower-top light collecting system and abeam-down light collecting system. The tower-top light collecting systemincludes a heliostat group and a receiver disposed on a tower top, andis a system which collects the light reflected by the heliostat group atthe receiver on the tower top. The beam-down light collecting systemincludes a heliostat group, a reflector disposed on a tower top, and areceiver disposed on the ground, and is a system which secondarilyreflects the sunlight, which has been primarily reflected by theheliostat group, and collects the light on the receiver.

Further, the tower-top light collecting systems are classified, on thebasis of the shape of the receiver (heat collector), into three types,namely, a flat receiver type, a cavity receiver type, and a cylindricalreceiver type. In the flat receiver type, a flat receiver (heatcollector) is arranged on a top part of a tower vertically andnorthwardly (in a case of the northern hemisphere), and heliostats arearranged only on the north side of the tower to collect the reflectionlight on the receiver on the tower. In the cavity receiver type, acavity receiver (heat collector) is arranged on a top part of a tower sothat an opening thereof faces northward and obliquely downward (in acase of the northern hemisphere), and heliostats are arranged only onthe north side of the tower to collect the reflection light on thereceiver on the tower. In the cylindrical receiver type, a cylindricalheat collector is arranged on a top part of a tower, and heliostats arearranged around the tower to collect the light reflected from therespective heliostats on the receiver on the tower.

On the other hand, according to the beam-down light collecting system,which includes a heliostat group, a reflector and a receiver, and is asystem in which the light primarily reflected by the heliostat group issecondarily reflected by the reflector on a top part of the tower andthe secondarily-reflected light is collected on the receiver arranged ona bottom part of the tower (on the ground), arranging the heliostatsaround the tower enables light collection from periphery of the tower(see Patent Documents 1 to 3).

In the beam-down light collecting system, two or more towers may bearranged at intervals among the heliostats that are dispersedly arrangedon the ground, and this is called as a multi-tower beam-down lightcollecting system.

Also, in the cylindrical receiver type tower-top light collectingsystem, two or more towers may be arranged at intervals among theheliostats that are dispersedly arranged on the ground, and this iscalled as a multi-tower tower-top light collecting system.

When comparing the functions of the multi-tower tower-top lightcollecting system and the multi-tower beam-down light collecting system,as for the multi-tower tower-top light collecting system, in a case inwhich the heliostats are continuously arranged in the east-westdirection as shown in FIG. 12 and the sun is on the east side forexample, there is a large difference in light collecting quantitybetween the east-side surface and the west-side surface of the receiver(heat collector) on the top part of the tower.

In such a case, on the east-side surface of the heat collector, thelight collecting quantity becomes deficient, with the result that theheat collecting efficiency decreases significantly. To the contrary, inthe multi-tower beam-down light collecting system, as shown in FIG. 13,the light from the heliostats disposed in any direction is, in theory,collected uniformly on the upper surface of the receiver that isdisposed on a bottom part of the tower. Therefore, it is possible tosuppress the decrease of the heat collecting efficiency due to theshortage of the light collecting quantity which is caused in themulti-tower tower-top light collecting system, whereby highheat-collecting efficiency is obtained.

For such reason, the multi-tower beam-down light collecting system isadvantageous as compared with the multi-tower tower-top light collectingsystem.

Nevertheless, the advantage of the multi-tower beam-down lightcollecting system is only in comparison with the heat collectingefficiency of the multi-tower tower-top light collecting system.

Patent Document 1: JP 2951297 B2 Patent Document 2: JP 2000-146310 APatent Document 3: JP 2004-37037 A DISCLOSURE OF THE INVENTION Problemthat the Invention is to Solve

While the multi-tower beam-down light collecting system is advantageousas compared with the multi-tower tower-top light collecting system, thisis only in comparison with the multi-tower tower-top light collectingsystem, and a problem that the invention is to solve is that there isroom for further improvement also in the multi-tower beam-down lightcollecting system.

Means for Solving the Problem

The most distinctive feature of the present invention is to select, in amulti-tower beam-down light collecting system, a tower toward which aheliostat reflects light, in accordance with according a position of thesun so as to increase light collecting quantity.

More specifically, the present invention is a solar light collectingmethod in a multi-tower beam-down light collecting system, having atower selection,

the multi-tower beam-down light collecting system being a system inwhich, in a field where a plurality of beam-down light collecting towersare present, light primarily reflected by a heliostat is secondarilyreflected by a reflector at a top part of one of the towers and iscollected on a receiver on the ground, and

in which the tower selection includes comparing, assuming that theheliostat in a given position receives sunlight and reflects thesunlight toward each of optionally selected two of the towers, a lightreceiving quantity on the receiver of each of the towers, and selectingone of the towers in which the light receiving quantity is relativelylarge to reflect the sunlight toward the one of the towers.

As a practically simple method, for example, the tower may be selectedsuch that, assuming that the heliostat in a given position receives thesunlight and reflects the sunlight toward each of the optionallyselected two of the towers, an angle formed by an directional vector ofincident light and a directional vector of reflection light seen fromthe heliostat is compared, the magnitude of the angle formed by thedirectional vector of the incident light and the directional vector ofthe reflection light seen from the heliostat is evaluated, and a towerwith respect to which the angle formed by the directional vector of theincident light and the directional vector of the reflection light seenfrom the heliostat is smaller is determined as the tower in which thelight receiving quantity is relatively large.

ADVANTAGE OF THE INVENTION

According to the present invention, in the multi-tower beam-down lightcollecting system, each of the heliostats that are dispersedly arrangedon the ground are made to select the tower toward which the reflectedsunlight is to be collected, whereby conversion efficiency of the solarenergy can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a basic structure of a beam-down lightcollecting system as a solar light collecting system using heliostats;

FIG. 2 is a diagram showing an example of a configuration of aheliostat;

FIG. 3 is a diagram showing an example of a configuration of areflector;

FIG. 4( a) is a plan view showing an example of a configuration of amulti-tower beam-down light collecting system, and FIG. 4( b) is asectional view taken along the line A-A in FIG. 4( a);

FIG. 5 is a diagram showing a relation between a tower to be selectedand a position of the sun;

FIG. 6 is a diagram showing an example in which a tower, toward whichsunlight is to be reflected, is selected based on the magnitude of anangle formed by the sun S and a neighboring tower seen from anheliostat;

FIG. 7 is a diagram showing an example in which a tower having largerlight receiving quantity on the receiver is selected from the respectivetowers;

FIG. 8 is a diagram showing a case in which two towers are lined on theeast and west as a calculation example of tower selection using a lightcollecting simulator;

FIG. 9 is a diagram showing a comparison of reflected energy quantitybetween when a tower toward which the sunlight is to be reflected isselected and when not selected;

FIG. 10 is a diagram showing a rate of the reflected energy quantityincreased by selecting the tower toward which the sunlight is to bereflected;

FIG. 11 shows the reflected energy quantity from a heliostat in a day,in which (a) is a diagram showing the reflected energy quantity in acase of a single-tower light collecting system, and (b) is a diagramshowing the reflected energy quantity in a case in which a towerselection is carried out successively such that an angle formed by areflector (upper focus) and the sun seen from a heliostat becomessmaller;

FIG. 12 is a diagram showing a light collection in a multi-towertower-top light collecting system; and

FIG. 13 is a diagram showing a light collection in a multi-towerbeam-down light collecting system.

EMBODIMENTS OF THE INVENTION

An object to select a tower such that light receiving quantity becomesthe largest when a heliostat receives sunlight of the sun in a givenposition is achieved by evaluating the magnitude of the light receivingquantity on a receiver of the tower, finding a relation between aposition of the sun and a tower to be selected, and controlling theheliostat.

Embodiment 1

A multi-tower beam-down light collecting system is a system in which, ina field where a plurality of beam-down light collecting towers arepresent, light primarily reflected by heliostats around each tower issecondarily reflected by a reflector on a top part of the tower tocollect the light on a receiver on the ground. FIG. 1 shows a basicstructure of a beam-down light collecting system as a solar lightcollecting system using heliostats. In FIG. 1, the beam-down lightcollecting system is configured by a combination of a group ofheliostats 1, 1, . . . dispersedly arranged on the ground and a tower 4including a reflector 2 and a receiver 3. The reflector 2 is areflection mirror disposed in a position of an upper focus at a top partof the tower 4, and the receiver 3 is disposed in a position of a lowerfocus at a bottom part of the tower 4 (on the ground) so as to face thereflector 2. The beam-down light collecting system is a system in whichthe sunlight primarily reflected by the heliostats 1 is secondarilyreflected by the reflector 2, thereby collecting the light on thereceiver 3.

The heliostat (a primary reflection mirror) 1 is, as shown in FIG. 2, adevice which can orient a mirror 5 in any direction to reflect thesunlight in the direction toward which the mirror 5 is oriented. Thereflector (a central reflection mirror, a secondary reflection mirror) 2is, as shown in FIG. 3, a device which reflects again the sunlight b1reflected from the heliostat 1 toward the receiver 3 by a mirror surface6. The reflector 2 may be a hyperboloid-of-revolution type, a segmenttype or the like. Further, the receiver 3 is a light collector forreceiving the collected light, and is classified into a flat type, acylindrical type, a cavity type, and the like based on its shape.

FIG. 4 shows an example of a configuration of the multi-tower beam-downlight collecting system. The multi-tower beam-down light collectingsystem is configured by a combination of the tower 4 and a group ofheliostats 1, 1 . . . around the tower 4. According to the presentinvention, the combination of the tower targeted by each heliostat andthe respective heliostats is not necessarily identified. Therefore, asan embodiment, the towers are arranged at certain intervals among groupsof heliostats 1, 1 . . . arranged dispersedly on the ground.

According to the present invention, each of the heliostats 1 selects,from some of the towers 4 set up nearby, a specified tower 4 such thatthe light receiving quantity on the receiver becomes the largest, andsends forth the reflected sunlight toward the reflector 2 of thespecified tower 4 that has been selected. This will be referred to as atower selection.

More specifically, the tower selection is a process in which, the lightreceiving quantity on the receiver 3 of the tower 4 is compared,assuming that the heliostat 1 in a given position receives the sunlightand reflects the sunlight toward each of optionally selected two towers4, 4, and the tower 4 in which the light receiving quantity isrelatively large is selected to reflect the sunlight toward that tower4.

FIG. 4( a) shows a relation between the heliostats 1 arrangeddispersedly on the ground and a tower to be selected by each group ofheliostats 1 at a given time. In FIG. 4( a), large circles show thetowers 4 each including the reflector 2 and the receiver 3, and smallcircles, triangles and rectangles therearound show the heliostats 1.Circular, triangular and rectangular marks indicated inside therespective large circles show that the tower is selected by theheliostats 1 of the same mark.

While the heliostats 1 are aligned in the longitudinal and lateraldirections in this drawing, a group of heliostats that targets the sametower forms a hexagonal formation, and the tower 4 which collects thelight is located in the lower right position therein. When focusingattention on the individual heliostats, each of the heliostats 1reflects the sunlight toward the selected tower in accordance with thetower selection, regardless of the distance to the tower as shown inFIG. 4( b), and the selected tower 4 collects the sunlight received onthe reflector 2 at the receiver 3 on the ground.

The collected sunlight is thermally stored, for example, in molten saltthrough the receiver (heat collector) and is used for various purposes.Alternatively, the collected sunlight is further collected by asecondary light collector called CPC and, thereafter, produces chemicalenergy fuel through an endothermic chemical reaction in a chemicalenergy conversion receiver (heat collector for chemical energyconversion).

In order to make the heliostat 1 reflect the sunlight toward theselected tower 4, the orientation of the heliostat 1 is controlled. Theorientation control is performed by a calendar method and/or a sensormethod described below.

(a) Calendar Method

Among the methods in which a directional vector of the heliostat iscalculated from coordinates of the heliostat, coordinates of a target,and a directional vector of the sun to control the orientation so as toface in that direction, the calendar method is a method in which thedirectional vector of the sun is calculated from the latitude, longitudeand the time. This calculation may be performed either independently foreach heliostat or on a computer which centrally controls the pluralityof heliostats.

(b) Sensor Method

The sensor method is a method of controlling the orientation of theheliostat using a reflection light sensor provided in each heliostat.Since this method is not influenced by the installation error of theheliostat or an error in a control mechanism, it can perform the controlwith good accuracy. However, when using the sensor method, the samenumber of sensors as the number of selectable towers is required foreach heliostat. Further, there is limit in a sensitivity range of thesensor. Therefore, the control based only on this method is difficult.Accordingly, this method is usually used in combination with thecalendar method.

An example of a heliostat orientation controlling method using thecalendar method will be described below. However, the heliostatorientation controlling method is not limited to this method. Inaccordance with the following steps, calculation is performed, in which,when seen from the heliostat, S is a directional vector of the sun and Fis a directional vector to a target point on the tower.

Step S1: In accordance with the following expression, a heliostatdirectional vector (normal vector) N is calculated.

N=(S+F)/|S+F|

Step S2: Since the orientation of the heliostat is controlled by anazimuth A and an elevation angle E, the respective values arecalculated.

E=a sin(N·z)

A=a tan(N·y/N·x) (Range of A is 0 degree to 360 degrees)

Step S3: Based on the values obtained in Step S2, the heliostat iscontrolled so as to be in the calculated orientation.Step S4: The above steps are repeated with change of the solardirectional vector, and the orientation of the heliostat is sequentiallychanged accordance with the change of the solar directional vector.

In the invention, assuming that the heliostat in a given positioncollects the light toward a given tower when the sun is in a givenposition, the quantity of light collected on a receiver of the giventower may be obtained, not necessarily through an actual measurement,but by finding a relation between the solar position and a tower to beselected in advance through calculation using a light collectingsimulator.

In the beam-down light collecting system, the sunlight reflected by theheliostat 1 is reflected again by the reflector 2 of the tower 4 and iscollected on the receiver 3. Nevertheless, the quantity of lightreceived on the heliostat 1 is not the same as the quantity of lightreceived on the receiver 3, and decreases due to various factors. Byperforming light collecting calculation, these factors are taken intoconsideration, and the quantity of light to be received on the receiver3 is obtained.

The light collecting calculation is performed, for example, by a raytracing method in which each light ray of the sun is traced one by onein accordance with the procedure of the following steps.

Here, the light ray is what puts three elements together, namely a passpoint (a point on the ray including a starting point and an end point)p, a directional vector v, and intensity e.

Procedure of light-collection calculating method (light-collectioncalculation with a given solar position)

Step T1: Tracing a light ray which is emitted from a given position onthe solar surface (because the sun is not a point light source but asurface light source) and reaches a given position (determination ofinitial light ray vector).Step T2: Determining whether the light ray hits a given heliostat(Cosine factor).Step T3: Determining whether the light ray is shielded, before reachingthe given heliostat, by another heliostat or other obstacles(Shadowing).Step T4: Reflecting the light ray by the heliostat (primary reflectedray) (Attenuation based on reflectance and cleanliness; Variation ofreflection angle due to mirror installation error, and the like)Step T5: Determining whether the primary reflected ray is shielded byanother heliostat or other obstacles (Blocking).Step T6: Determining whether the primary reflected ray hits a reflector(Spillage in reflector).Step T7: Reflecting the primary reflected ray by a central reflectionmirror (secondary reflected ray) (Attenuation based on reflectance,cleanliness and air; Variation in reflection angle due to mirrorinstallation error, and the like)Step T8: Determining whether the secondary reflected ray enters areceiver opening (Spillage in receiver).Step T9: The secondary reflected ray reaching a receiver (Attenuation byair).Step T10: Repeating the above steps

In the invention, the processing of comparing the magnitude of lightcollecting quantity, assuming that a given heliostat receives the lightfrom the sun in a given position, between receivers of two optionalneighboring towers, and selecting the tower in which the lightcollecting quantity is larger, may be performed by means of a simulator.When selecting the tower using a simulator, a light collecting simulatoris used, receiver light-receiving-quantity calculation, whole-skydivision for tower selection, and receiver light-receiving-quantitycomparison are performed and, thereafter, the tower selection isperformed based on results of these processing. In the tower selection,the tower selection when the sun is in a given position is performed,and the sunlight is collected to the selected tower.

More specifically,

the receiver light-receiving-quantity calculation is a processing ofcalculating the light receiving quantity assuming that the heliostatreflects the sunlight toward each tower when the sun is in a givenposition;

the whole-sky division is a processing of, based on the result of thereceiver light-receiving-quantity calculation, dividing the whole sky bya boundary, on a position of which the respective light receivingquantities of the adjacent towers are the same; and

the receiver light-receiving-quantity comparison is a processing ofcomparing the light quantity to be received the receiver in each area ofthe whole sky divided by the whole-sky division, and indicating thetower in which the light receiving quantity is larger. In the towerselection, based on the result of the receiver light-receiving-quantitycomparison, the tower determined to be large, when the sun is in a givenposition, in light receiving quantity is selected, the orientation ofthe heliostat is controlled so that the heliostat reflects the sunlighttoward the selected tower, and the sunlight received by the heliostat isreflected toward the selected tower.

As a calculation example of the tower selection by means of the lightcollecting simulator, a case where two towers are arranged east and westwill be described below (see FIG. 5).

Calculation Conditions

Tower position: Tower 4L (−150, 0, 100), Tower 4R (150, 0, 100)Heliostat position: (50, 50, 0)Setting of heliostat focal distance: 150 m(Distance at which the reflection light from the heliostat focuses animage: the heliostat forms a pseudo concave mirror by a plurality offacet mirrors.)

(1) Receiver Light-Receiving-Quantity Calculation:

The light receiving quantity of the receiver in each tower when the sunis in a position of a given solar elevation and a given solarorientation has been found by calculation and a result of table 1 hasbeen obtained. In the table 1, Azimuth represents a solar orientationangle (deg), Elevation is a solar elevation angle (deg), H1 refrepresents the light reflection quantity in case that the light iscollected to the tower 4L, H1rec represents the receiver light receivingquantity in case that the light is reflected by the tower 4L, H2refrepresents the heliostat light reflection quantity in case that thelight is reflected to the tower 4R, and H2rec is the receiver lightreceiving quantity in case that the light is reflected to the tower 4R.Further, in the table 1, the east is taken as an origin (0 degree), thenorth is taken as 90 degrees, the west is taken as 180 degrees, and thesouth is taken as 270 degrees. Further, the heliostat light-refectionquantity is described as reference.

TABLE 1 Relation between solar position and light collecting quantity ineach tower Azimuth Elevation H1ref H1rec H2ref H2rec  0 10 387.107111.348 818.153 721.408  0 30 722.318 264.046 1552.12 1373.04  0 50996.741 476.059 1605.81 1420.28  0 70 1211.01 605.066 1575.43 1392.81  090 1381.06 732.076 1497.86 1313.85  30 10 400.628 113.258 1232.861078.77  30 30 732.622 270.056 1473.2 1298.44  30 50 1000.5 482.4211517.26 1339.68  30 70 1205.42 605.175 1532.73 1350.64  30 90 1382.77733.951 1489.51 1308.01  60 10 637.249 240.888 1066.38 891.28  60 30870.539 372.101 1345.8 1159.97  60 50 1099.73 555.989 1432.51 1252.26 60 70 1256.74 671.049 1481.53 1298.95  60 90 1379.92 731.819 1494.191310.74  90 10 537.104 259.238 622.865 479.06  90 30 1152.33 582.081160.49 926.496  90 50 1254.83 675.23 1313.84 1117.27  90 70 1329.92729.002 1413.87 1229.91  90 90 1386.68 736.765 1489.63 1307.18 120 101122.59 614.479 686.932 359.932 120 30 1368.38 740.107 1006.47 690.428120 50 1410.18 775.2 1208.37 976.036 120 70 1411.49 774.331 1368.61175.46 120 90 1380.99 733.381 1494.17 1311.5 150 10 1298.5 726.513616.878 271.147 150 30 1515.53 818.465 928.588 580.749 150 50 1518.28825.833 1165.5 908.623 150 70 1472.55 802.038 1352.93 1157.21 150 901382.05 731.96 1490.44 1306.51 180 10 818.153 486.185 596.38 319.166 18030 1532.76 820.542 980.201 646.047 180 50 1578.88 846.465 1191.85957.465 180 70 1512.52 816.713 1363.41 1168.66 180 90 1384.49 735.4771490.01 1307.22 210 10 1392.83 763.178 819.831 571.754 210 30 1607.49856.642 1118.98 885.979 210 50 1589.52 848.509 1282.89 1087.7 210 701509.99 813.816 1406.05 1218.9 210 90 1381.45 733.063 1495.52 1312.11240 10 1299.37 719.02 1031.55 843.348 240 30 1519.1 821.573 1296.541106.12 240 50 1520.06 823.841 1404.77 1226.27 240 70 1472.05 803.3541467.1 1286.72 240 90 1382.28 734.209 1489.57 1306 270 10 655.82 366.929765.71 672.311 270 30 1352.06 730.564 1453.26 1276 270 50 1394.38765.643 1523.65 1342.85 270 70 1409.34 772.71 1530.19 1346.49 270 901384.16 734.556 1492.39 1308.74 300 10 905.019 443.264 1335.93 1176.92300 30 1138.19 573.582 1573.53 1390.71 300 50 1242.71 661.241 1601.431415.84 300 70 1328.35 724.13 1568.51 1384.77 300 90 1383.42 731.7671495.13 1311.45 330 10 585.233 221.102 1367.5 1206.57 330 30 897.412382.687 1618.07 1430.86 330 50 1092.07 552.002 1634.83 1444.97 330 701256.41 662.566 1586.16 1400.28 330 90 1383.15 733.239 1499.22 1315.26

(2) Whole-Sky Division:

In this processing, from the result of the table 1, regarding therelation between the tower to be selected and the solar position, thesolar elevation at which the receiver light receiving quantity (H1rec)of the tower 4L and the receiver light receiving quantity (H2rec) of thetower 4R become the same is found for each solar orientation, and thewhole sky is divided by a boundary line connecting the obtainedelevation positions. Here, from the solar elevation and the lightreceiving quantity (Table 1), the solar elevation at which the lightcollecting quantity of each tower becomes the same is found for eachsolar orientation. The results are shown in Table 2.

TABLE 2 Relation between solar orientation and solar elevation at whichlight collecting quantities of towers 1 and 2 become the same AzimuthEven ref Even rec  0 0    0     30 0    0     60 0    0     90 0    0   120 75.49625 33.96615 150 80.49252 44.83379 180 81.7119  42.22403 21079.53534 27.34219 240 70.88204 0    270 0    0    300 0    0    330 0   0   

The results in the Table 2 are expressed in polar coordinate in whichthe zenith is taken as zero, as shown in Table 3 (In the elevationangle, a ground level is taken as 0 degree and the zenith is taken as 90degrees. In the polar coordinate, the zenith is taken as 0 degree andthe ground level is taken as 90 degrees.) Further, the table 3 isplotted with the polar coordinate as shown in FIG. 6.

Here, a circle shows the whole sky. A center is the zenith and thecircumference becomes the ground level. When the sun lies in theposition of a dotted line, the receiver light receiving quantity in eachtower in case that the heliostat reflects the sunlight toward each towerbecomes equal. When the sun lies in the position of a solid line, thereflection quantity from the heliostat in case that the heliostatreflects the sunlight toward each tower becomes equal. In the actualcase, by shortening each interval of the azimuth and the elevation anglein this calculation, more exact figure can be created.

TABLE 3 Relation between solar orientation and solar elevation at whichlight collecting quantities of towers 4L and 4R become the same (polarcoordinate) Azimuth Even ref Even rec  0 90     90      30 90     90     60 90     90      90 90     90     120 14.50375 56.03385 150  9.50747845.16621 180  8.288104 47.77597 210 10.46466 62.65781 240 19.1179690     270 90     90     300 90     90     330 90     90    

(3) Receiver Light-Receiving-Quantity Comparison:

The light receiving quantity (H1rec, H2rec) in relation to the solarazimuth is compared between the respective towers, and the magnitude ofthe light receiving quantity is evaluated. Firstly, from the Table 1, itis found that: when the azimuth is 0, 30, 60, 90, 240, 270, 300, or 330degrees, the light receiving quantity H1rec is always smaller than thelight receiving quantity H2rec. Namely, when the solar orientation is inthis range, it is better that the tower 4R is always selected. Here, forconvenience sake, the elevation at which the light receiving quantitybecomes the same is taken as 0 degree.

Next, from the Table 1, it is found that: when the azimuth is 120, 150,180, or 210 degrees and when the solar elevation is low, the lightreceiving quantity Hlrec is larger than the light receiving quantityH2rec; and when the solar elevation is high, the light receivingquantity H1rec is smaller than the light receiving quantity H2rec.Considering this result, the tower having the larger light receivingquantity in each of the divided areas of the whole sky is as shown inFIG. 7.

(4) Tower Selection

When the sun is in a given position, a tower to be selected can beselected by means of FIG. 7.

Further, though the light collecting quantity decreases due to variousfactors in the light calculation, it is also possible to perform simplythe tower selection in consideration of the decrease due to only acosine factor of their factors. In this case, it can be said that thetower selection is processing of performing control so that an angleformed by the sun and each upper focus becomes small.

Namely, as shown in FIG. 8, comparison is performed between an angle θ1formed by the sun S and a neighboring tower 4 a when seen from a givenheliostat 1, and an angle θ2 formed by the sun S and another neighboringtower 4 b. In case that θ1<θ2, the tower 4 a is selected, whereby thereflection amount from the heliostat is made substantially largest andthe effective use of solar energy can be made. However, the lightreceiving quantity of the receiver is determined by other many factors.Therefore, in this case, compared with the case where the towerselection is rigorously performed by means of the light collectingsimulator, the light collecting quantity decreases.

Next, regarding how effective such the tower selection in considerationof the decrease due to only such the cosine factor is, in themulti-tower beam-down light collecting system, in the example where sucha tower that the angle formed by the sun and each tower seen from theheliostat becomes small is selected and the sunlight is collected to theselected tower, the calculation has been performed using the model inFIG. 5 described before. In FIG. 5, an X-axis represents an east-westdirection, and a Y-axis represents a zenith direction. It is assumedthat: on an X-Y plane, the sun rises from the east at 6:00 a.m., passesthrough the zenith at 12:00, and sinks at 6:00 p.m.

Here, the direct normal irradiance (DNI) is taken as 1.0 kW/m², the twotowers are set respectively in the position of X=150 m and in theposition of X=−150 m, and the positions (heights of upper focus) of thereflectors in the both towers are taken as Y=100 m. The area of a mirrorper heliostat is taken as 1.0 m², and the thirty heliostats have beenarranged between the both towers. Blocking and shadowing among theheliostats are not taken into consideration.

FIG. 9 shows the reflected energy quantity of the heliostat in a day.O-mark shows the reflected energy quantity in case that the heliostatcollects the light to the left tower, and □-mark shows the reflectedenergy quantity in case that the heliostat collects the light to theright tower. Δ-mark shows the reflected energy quantity in case that thetower in which the angle formed by the reflector (upper focus) and thesun which are seen from the heliostat becomes small is selected asneeded. Further, FIG. 10 shows the rate of the reflected energy quantityincreased by performing the tower selection. As clear from theseresults, it has been found that the reflected energy quantity of eachheliostat in a day increases by 5% to 22% due to the tower selection,compared with the case of the single-tower light collecting system. Atthe same time, it has been found that the heliostat located in anintermediate point between the two towers is highest in rate of itsincrease.

Further, FIG. 11 shows the reflected energy quantity of the heliostat ina day. In this figure, the towers are located on both sides of arectangular filed. FIG. 11( a) shows the reflected energy quantity incase of a single-tower light collecting system, and (b) shows thereflected energy quantity in case that the tower in which the angleformed by the reflector (upper focus) and the sun which are seen fromthe heliostat becomes small is selected as needed. It has been foundfrom this figure that the area of a region where the reflection quantityfrom one heliostat becomes 10.5 kWh and more is about 13 times, comparedwith that in the single-tower light collecting system.

Than the above simple tower selection, the tower selection by means ofthe result of the elevation calculation and the result of the lightreceiving quantity comparison in FIGS. 6 and 7 becomes more suitable.Namely, as clear from FIG. 6, in comparison between a case (solid line)where the tower light receiving quantity becomes simply equal inconsideration of only the angle and a case (dashed line) where the towerlight receiving quantity becomes equal by performing the elevationcalculation, the area of region surrounded by the dashed line where thelight is reflected toward the tower 4R is larger than that surrounded bythe solid line, the tower selection is appropriately performed there andthe tower light receiving quantity becomes larger.

As described above, in the invention, in case that the sun is in a givenposition, the light collecting calculation is performed and the towerselection is performed. In case that the invention is actually appliedto the control of heliostat, such the calculation is performed inadvance and its calculation result can be used also in control of theheliostat operation. Further, by simultaneously carrying out thehigh-speed calculation processing of the light collecting simulatorduring the heliostat operation, when the light of the sun lying in thesolar position at that time is received, during the operation, themagnitude of the light collecting quantities of the receivers in the twooptional towers near the heliostat is evaluated, whereby the processingof selecting the tower in which the light collecting quantity is largemay be performed. When the tower selection is performed bysimultaneously performing the high-speed calculation processing of thelight collecting simulator, the receiver light-receiving-quantitycalculation is performed by the light collecting simulator, and thetower selection is performed on the basis of the result of the receiverlight-receiving-quantity comparison. Therefore, the whole-sky divisionmay be applied as needed.

INDUSTRIAL APPLICABILITY

The sunlight, as a renewable energy source, has an enormous quantity ofenergy, and is a clean energy source which has no environment pollution.The sunlight enables fuel production which utilizes the concentratedsolar thermal energy in endothermic reaction of chemical reaction, andthe stable supply of the generated electric power by concentrating thethin solar energy as the solar thermal power generation system. Further,by applying the sunlight to technology of synthesizing methanol fromhydrogen and carbon monoxide which have been manufactured by coalgasification and natural gas steam reforming, it is possible tomanufacture methanol of which heat quantity is 6-10% or more larger thantotal heat quantity of coal and methane of raw materials, and thesunlight is greatly expected as what can significantly reduce emissionof carbon dioxide in the methanol manufacturing process.

DESCRIPTION OF REFERENCE SIGNS

-   -   1 Heliostat    -   2 Reflector    -   3 Receiver    -   4 Tower    -   5 Mirror    -   6 Mirror Surface

1. A solar light collecting method in a multi-tower beam-down lightcollecting system, comprising a tower selection, wherein the multi-towerbeam-down light collecting system is a system in which, in a field wherea plurality of beam-down light collecting towers are present, lightprimarily reflected by a heliostat is secondarily reflected by areflector at a top part of one of the towers and is collected on areceiver on the ground, and wherein the tower selection comprisescomparing, assuming that the heliostat in a given position receivessunlight and reflects the sunlight toward each of optionally selectedtwo of the towers, a light receiving quantity on the receiver of each ofthe towers, and selecting one of the towers in which the light receivingquantity is relatively large to reflect the sunlight toward said one ofthe towers.
 2. The solar light collecting method in the multi-towerbeam-down light collecting system as set forth in claim 1, wherein, inthe tower selection comprises comparing, assuming that the heliostat inthe given position receives the sunlight and reflects the sunlighttoward each of the optionally selected two of the towers, an angleformed by an directional vector of incident light and a directionalvector of reflection light seen from the heliostat, evaluating themagnitude of the angle formed by the directional vector of the incidentlight and the directional vector of the reflection light seen from theheliostat, determining that a tower with respect to which the angleformed by the directional vector of the incident light and thedirectional vector of the reflection light seen from the heliostat issmaller is the tower in which the light receiving quantity is relativelylarge, and selecting said tower.
 3. The solar light collecting method inthe multi-tower beam-down light collecting system as set forth in claim1, wherein the tower selection is performed based on results of areceiver light-receiving-quantity calculation, a whole-sky division, anda receiver light-receiving-quantity comparison, wherein the receiverlight-receiving-quantity calculation comprises calculating a lightreceiving quantity in each receiver assuming that a given heliostatreflects the sunlight toward each tower when the sun is in a givenposition, wherein the whole-sky division comprises finding, based on theresult of the receiver light-receiving-quantity calculation, a boundaryline on the whole sky along which the light receiving quantity in eachreceiver becomes the same assuming that the given heliostat reflects thesunlight toward each tower, and dividing the whole sky by the boundaryline, wherein the receiver light-receiving-quantity comparisoncomprises, with respect to each area of the whole sky divided by thewhole-sky division, comparing the light receiving quantity to bereceived by the receiver of each tower assuming that the heliostatreflects the light toward each tower, and evaluating the magnitude ofthe light receiving quantity to be received by the tower, and whereinthe tower selection comprises determining, based on the result of thereceiver light-receiving-quantity comparison, which tower is to beselected when the sun is in the given position, controlling anorientation of the heliostat so that the heliostat reflects the sunlighttoward the tower that is determined to be large in light receivingquantity, and reflecting the sunlight received by the heliostat towardthe selected tower.
 4. The solar light collecting method in themulti-tower beam-down light collecting system as set forth in claim 3,wherein the receiver light-receiving-quantity calculation comprisescalculating the light receiving quantity to be received by the receiverof each towers assuming that the heliostat reflects the sunlight towardeach tower when the sun is in a position of a given solar orientationand a given solar elevation.
 5. The solar light collecting method in themulti-tower beam-down light collecting system as set forth in claim 3,wherein the receiver light-receiving-quantity calculation comprisescalculating the light receiving quantity to be received by the receiverof each towers assuming that the heliostat reflects the sunlight towardeach tower when the sun is in a position of a given solar orientationand a given solar elevation; and wherein the whole-sky divisioncomprises finding a solar elevation at which the light receivingquantity to be received by the receiver of each towers adjacent to eachother becomes the same in the given solar orientation to find theboundary line along which the light receiving quantity to be received bythe receiver of each towers adjacent to each other becomes the same, anddividing the whole sky by the boundary line.
 6. The solar lightcollecting method in the multi-tower beam-down light collecting systemas set forth in claim 1, wherein the tower selection is performed basedon results of a receiver light-receiving-quantity calculation and areceiver light-receiving-quantity comparison; wherein the receiverlight-receiving-quantity calculation comprises calculating the lightreceiving quantity to be received by the receiver of each tower assumingthat a given heliostat reflects the sunlight toward each tower when thesun is in a given position; the receiver light-receiving-quantitycomparison comprises comparing, based on the result of the receiverlight-receiving-quantity calculation, the light receiving quantity to bereceived by the receiver of each tower assuming that reflection is madetoward each tower; and the tower selection comprises determining, basedon the result of the receiver light-receiving-quantity comparison, whichtower is to be selected, controlling an orientation of the heliostat sothat the heliostat reflects the sunlight toward the tower that isdetermined to be large in the light receiving quantity to be received bythe receiver, and reflecting the sunlight received by the heliostattoward the selected tower.
 7. The solar light collecting method in themulti-tower beam-down light collecting system as set forth in claim 1,wherein two or more towers are arranged at intervals among a group ofheliostats dispersedly arranged on the ground, and each of theheliostats selects, in accordance with the tower selection, a specifiedtower such that the light receiving quantity to be received by thereceiver becomes the largest.